The present invention relates to a microwave switch suited to a communications apparatus and operable in a microwave band or a millimeter-wave band and, more particularly, to a microwave switch with mechanical contacts which cuts down power consumption and heat generation and operates with desirable reliability and a minimal switching time.
Among switches applicable to the microwave band, those which are much in demand and of prime importance are the switches adapted to select either a regular equipment or a protection equipment at a communications station. Switches for such an application are required to feature small loss and high isolation between non-connected terminals and, for this reason, mostly comprise switches having mechanical contacts instead of those using semiconductive switching elements. As generally accepted, the microwave band covers frequencies from 1 GHz to over 10 GHz, while frequencies higher than 18 GHz are usually referred to as a quisemillimeter-wave or millimeter-wave band. What is requied of those switches which are designed for high frequency applications is that they entail a minimum of discontinuity in the signal transmission path and, hence, their freedom in regard to the contact structure is limited.
Traditionally, the mechanical contacts of such microwave switches were driven by electromagnetic actuators. An electromagnetic actuator is an electromechanical energy transducer which is called a solenoid and has long been used to show acceptable performance and reliability. Today, however, electromagnetic actuators cannot meet the state-of-the-art demands, particularly the ever increasing demands for small-size configuration and low power consumption in communications apparatus art as well as for a shorter switching time which is concomitant with the increasing trend toward digital communications.
Specifically, the problem with electromagnetic actuators is the power consumption and heat generation due to copper or ohmic losses of their windings. This problem has almost no chance to be provided with a solution. Although power consumption may be cut down by using a winding having a relatively large diameter, such an approach would result in slower switching and, in addition, a more bulky configuration which is contradictory to the demand for reduction in size. Another possible approach for power saving is reducing the amount of motion of a movable part of the actuator (hereinafter refered to as an armature). However, such would translate into a decrease in contact gap and, thereby, a decrease in isolation beyond a required one. Further, while reducing the force exerted by the actuator may lead to a decrease in power consumption, it would bring about shortage in contact pressure which in turn would limit the reliability of operation.
Another factor which limits the applicable range of a microwave switch used with an electromagnetic actuator is the switching speed. The switching speed cannot be enhanced beyond a certain limit partly because the electric time constant is large due to the use of a winding having a substantial inductance, and partly because the mechanical time constant is also large due to the use of an armature made of a magnetic material which has a large mass and inertia moment, and a relatively weak spring. In parallel with the recent transition of the communications principle from analog to digital, the switching time of microwave switches have come to be discussed as a critical problem. In digital communications, omission of signals due to switching is apt to occur and, in this respect, the switches should operate as fast as possible. Switches of the kind using PIN diodes and other semiconductors, although operable at high speeds, involve great insertion losses and low isolations and are, in many cases, unusable.